1. Field of the Invention
The present invention generally relates to a method of installing a foundation for an offshore wind turbine and a template for use herein.
2. Description of the related art
In an installation of offshore facilities, such as wind power installations, offshore platforms, submerged water-driven turbine installations and the like, foundations are often provided by a plurality of columns or piles driven into the seafloor. For example, a supporting structure of a wind power installation is often constituted by a cylindrical tower segment which may be coupled to a foundation in the ground. Independent of the kind of offshore facility under consideration, the stability of an offshore facility relies heavily on the support provided by the foundation. Foundations for offshore facilities, such as for wind power installations, are planned and constructed based on thorough analyses of water depths at the installation site and soil conditions of the seafloor at the installation site. In the case of wind power installations, further issues are to be considered, such as turbine specifications including nacelle weight, revolving speeds and more. It is therefore easy to see that planning and constructing an offshore foundation is a complex task where any failure has to be excluded for not posing a risk on the stability of the foundation.
In general, two types of foundations are used, gravity based foundation and jacket foundation. A conventional gravity based foundation includes a concrete cylindrical/conical support structure which is held in place by its own weight. The jacket foundation is a steel structure with typically four legs connected to each other with braces. Commonly, the legs are grouted to piles which are driven into the sea soil. In comparison with gravity based foundations, jacket foundations are easily transported to the installation site.
Further, gravity based foundation have mostly been used for smaller wind turbines in shallow near-shore projects with rocky sea floors. For larger turbines and deeper waters in general jacket foundations are preferred over gravity based foundation. For assuring stability, installation of piles in a seafloor is to be carefully planned and the piles are installed in accordance with a predetermined installation scheme. Herein, reliability of the foundation depends, besides the quality of the predetermined installation scheme, on the accurate realization of the predetermined installation scheme and deviations may result in a structurally weakened foundation such that accurate alignment of piles is of great importance. Particularly, a relative location of a second pile relative to a first and an orientation of the piles relative to a vertical reference orientation are important parameters based on which the stability of a foundation is determined. It is with regard to these parameters that alignment is to be achieved as misalignment may not allow safe carrying of loads that are imposed on the foundation.
During installation of piles, alignment is conventionally achieved by means of a template according to which a geometric pattern of piles may be installed in a seafloor. However, due to possible unevenness of the seafloor at the installation site, misalignment of piles may be caused by the template adopting a leveling position deviating from a reference leveling position, usually a horizontal level.
Document EP 2354 321 A1 shows a framed template for providing an offshore foundation being positioned by a jack-up platform. Herein, the framed template is lowered along spud poles towards the seafloor and piles are driven into the seafloor through sleeve guide members of the template. However, a fast installation of piles is not possible as the jack-up platform is to be installed at the installation site requiring the spud poles to be fixed to the, seafloor. Furthermore, particularly at great depths and rough sea conditions, usage of the jack-up platform may not be possible, while an accuracy in the orientation of the framed template depends on the orientation of the spud poles such that any misalignment of the spud poles causes the framed template to be misaligned.
Document GB 2469190 A shows a submerged platform with a drilling machine and telescopic legs for adjusting the platform to a horizontal position such that a column or pile may be anchored to the seafloor at a predetermined position. However, the platform may be subjected to displacement relative to the seafloor such that misalignment of the platform relative to a predetermined installation site may be caused.
Document CN 200971492 shows a method for installing an undersea drilling base plate on the seabed.
Therefore, it is an object of the present invention to ensure accurate alignment of piles when installing an offshore foundation.